The present invention relates to high strength core-sheath monofilaments for technical applications, possessing high dimensional stability and abrasion resistance and very good heat and hydrolysis resistance, and also to technical products manufactured therefrom, in particular paper machine fabrics, fabrics for screen printing and for technical filter materials. The core-sheath monofilaments of this invention have a polyester core and a sheath comprising a mixture of a thermoplastic polyester and a thermoplastic elastomeric polyurethane.
Monofilaments for technical applications are in most cases subjected to high mechanical stresses in use. In addition there are in many cases thermal stresses and stresses due to chemical and other environmental effects to which the material must offer adequate resistance. Under all these stresses, the material has to have good dimensional stability and constancy of the stress-strain characteristics over prolonged use periods.
An example of technical applications requiring a combination of high mechanical, thermal and chemical resistance is the use of monofilaments in paper machine fabrics, in particular in the Fourdrinier. This use requires a monofilament material having a high initial modulus and a high breaking strength, good knot and loop strength and high abrasion resistance to withstand the high stresses and ensure adequate fabric life. For use in the drying section of the paper machine, the monofil, as well as meeting these parameters, is additionally acquired to have a high hydrolysis resistance.
The monofilament material has to meet similar requirements for use in screen printing fabrics, which should have long service lives under the constant stress of the high pressure of the squeegee, the hydrolytic attack of aqueous dye pastes and the action of high energy actinic light sources. The dimensional stability requirements of screen printing fabrics are particularly high to make possible the production of multicolor prints which are in register.
At present, paper machine fabrics for the forming and drying sections are predominantly fabricated from polyethylene terephthalate monofilaments in warp and weft. These fabrics have the disadvantage of becoming longer in the transport direction in the course of the fabric life in the paper machine and therefore require retensioning.
Screen printing fabrics are these days fabricated from relatively fine-denier monofilaments of polyethylene terephthalate or polyamide in warp and weft. The main disadvantage of polyamide fabrics is the high water regain, which has an adverse effect on the elasticity, which screen printing fabrics must possess to a very high degree; polyester screen printing fabrics have poorer elasticity characteristics from the start. As a result, such known screen printing fabrics achieve only relatively short service lives.
There has never been a shortage of attempts to produce synthetic monofilament materials suitable for durable paper machine fabrics and screen printing fabrics. However, the requirements which these technical products have to meet are so varied that hitherto only partial solutions have been achievable in this field. For instance, it is known to fabricate paper machine fabrics using monofilaments of polyphenylene sulfides. This material does possess very good mechanical stability combined with excellent hydrolysis stability. However, it has a markedly low resistance to actinic radiation, so that such a monofilament material is completely unsuitable for making screen printing fabrics. The object of using this polymer to prepare monofilaments which are usable in both technical fields has therefore not been achieved. Japanese Laid-Open Patent Specification No. 45741 (1991) discloses making screen printing fabrics from polyethylene naphthalate monofilaments which, owing to their higher modulus of elasticity (initial modulus), are said to be less prone to slackening. However, these filaments show an abnormal tendency to fibrillate on weaving. Japanese Laid-Open Patent Specification No. 5209 (1993) discloses core-sheath monofilaments intended for making screen printing fabrics. The core of these filaments is poly(ethylene 2,6-naphthalate), the sheath is polyethylene terephthalate or modified polyethylene terephthalate. The modified sheath polyester may contain for example isophthalic acid, adipic acid or sebacic acid radicals or longer-chain diol radicals such as diglycol, butanediol or polyethylene glycol radicals, in which case the polyethylene glycol radicals may have a molecular weight of about 600 to 1500. According to the illustrative embodiment, the sheath polyester may be for example a polyethylene terephthalate modified with 8% by weight of polyethylene glycol radicals. This amount of modifier is sufficient to influence the strength properties and the melting characteristics, but falls short of conferring elastomeric properties on the polyester.
It is also known that it is possible to produce polyester fibers having very different mechanical and textile properties. More particularly, it is possible, by variation of the spinning and stretch drawing and relaxation conditions, to use polyethylene terephthalate to produce monofilaments which cover a wide spectrum of the properties relevant for technical monofilaments.
However, the efforts to obtain a monofilament material which simultaneously combines the high dimensional stability, abrasion resistance and hydrolysis resistance required for paper machine fabrics with the high resistance to actinic light required for making screen printing fabrics have so far not been wholly successful.
In the desire to find a polyester fiber which is suitable for as many technical applications as possible there has been no shortage of attempts either to replace polyethylene terephthalate by other polyester building blocks and by copolyesters. Alternative polyesters which have already been investigated employ for example polyethylene naphthalate and copolyesters of 4,4'-biphenyldicarboxylic acid and 2,6-naphthalenedicarboxylic acid, as described for example in European Patent Application No. 202,631. Fibers formed from 4,4'-biphenyldicarboxylic acid and 2,6-naphthalenedicarboxylic acid are proposed in WO 93/02122. These fibers have a high longitudinal strength and a high modulus when spun with a high drawdown without any further afterstretch-drawing. However, the usefulness of this material for the production of monofilaments, in particular for making paper machine and screen printing fabrics, has to be doubted, since it is known from experience that a high modulus is generally accompanied by low transverse strength.
A copolyester of 4,4,-biphenyldicarboxylic acid and 2,6-naphthalenedicarboxylic acid and ethylene glycol which is said to be suitable for producing tire cord is described in Japanese Patent Application 50-135,333. This reference reveals that such a copolyester must not contain more than 20 mol % of 4,4'-biphenyldicarboxylic acid, since otherwise its initial modulus and its softening temperature decrease too much. This statement is supported in the reference by illustrative embodiments which show that the softening temperature, which is 275.degree. C. for pure polyethylene naphthalate, drops to 238.degree. C. for a copolyester containing about 25 mol % of 4,4,-biphenyldicarboxylic acid. It is also known that polymers of 4,4'-biphenyldicarboxylic acid crystallize extremely rapidly. This is another reason why the manufacturability of monofilaments using this raw material has to be doubted, since an overly rapid crystallization leads to early embrittlement of the monofilaments even during the actual manufacturing process, so that they break before adequate orientation has been achieved. German Patent Application P-43 28 029.3 likewise proposes producing monofilaments essentially from a mixture of poly(ethylene 2,6-naphthalate) and poly(ethylene biphenylene-4,4'-dicarboxylate). A further proposal for producing paper machine fabrics is found in German Patent Application P-44 10 399.9. According to this proposal, the fabrics shall be woven from monofilaments spun from an abrasion-resistant polyester mixture comprising a mixture of a thermoplastic polyester and a thermoplastic polyurethane. There is no mention in said patent application of the use of core-sheath monofilaments.